1. Field of the Invention
The present invention relates to an image data processing apparatus, an image data processing method, and an image data processing program.
2. Description of Related Art
Conventionally, an image print process inputs image data generated from an image represented in multiple gradations of dot matrix pixels. The error diffusion method is used to diffuse a gradation error of each pixel to an unconverted pixel and convert the image into image data represented by presence or absence of dot formation corresponding to each pixel. The error diffusion method ensures good image quality because of no errors, but requires processing time for calculating and distributing errors. To solve this, a blocking process is performed on the basis of a block comprising 2×2 pixels. The blocking process diffuses a gradation error for the entire target block to unconverted pixels in another block and converts image data corresponding to pixels in the target block at a time. The blocking process can speed up conversion of image data. Compared to the pixel-based conversion, however, the blocking process decreases the image resolution and degrades the image quality.
There is an area that provides high image brightness and is capable of maintaining high image quality after a block-based conversion process. Such area is defined as satisfying a given block process condition. It is determined whether or not to satisfy the block process condition based on gradation values and the like for all pixels in the target block. When it is determined that the block process condition is true, a process is performed to convert image data for pixels in the target block at a time. When it is determined that the block process condition is false, a process is performed to unconditionally convert image data for respective pixels in the target block. As a result, a dot is formed in units of blocks only in such an image data area that provides a very small density for forming dots, hardly presents degradation of the resolution, and is capable of maintaining the high image quality. The high quality is maintained by speeding up the conversion of image data to a certain extent.
There has been a demand for the above-mentioned prior art to further speed up the conversion of image data.
According to the prior art, different dot patterns are formed depending on whether an area is subject to the block-based process or the pixel-based process. An unintended design may result.
Further, according to the prior art, high resolution printing requires the block-based image data conversion in units of 2×2 pixels. This is seemingly a smaller processing unit than that for low resolution printing. The processing speed is not fully accelerated.
Since the prior art unconditionally determines block process conditions, it has been impossible to print images by increasing the processing speed even if the image printing does not require high quality. As a result, it has been felt to spend more time than needed until an image printout is produced. Accordingly, there has been a demand for accelerating the image data conversion and fast creating an image printout.